Flow injection analysis (FIA) performed with an atmospheric pressure ion (API) source interfaced to a mass spectrometer (MS) is a common method for introducing sample into an API MS apparatus. Typically, API MS FIA is performed by connecting a sample injector valve in-line between a solvent or solution delivery system and an inlet probe to an API source. When the injector valve is switched to the load position, sample solution is loaded into the injector valve through an injector needle while the separate solvent delivery system provides liquid flow to the API source probe through a separate channel in the injector valve. The injector valve is then switched so that the loaded sample solution, usually contained in a tubing loop or channel, is connected to the solvent delivery channel and the sample solution flows into the API source through the API probe at a flow rate set by the solvent delivery system. The injector valve is generally connected to the API source probe with a transfer line or tube. The sample is introduced into the injector valve through an injector needle that is connected to a fluid reservoir or transfer line. The injector needle may be connected to a syringe for manual injection, a syringe configured in an autoinjector or transfer tube or line that is connected to a remote fluid delivery means configured in an autoinjector. In some commercially available autoinjectors, the injector needle is attached directly to a syringe. The syringe and injector needle are loaded, emptied and positioned by the autoinjector apparatus. The autoinjector moves the injector needle into a vial or container holding sample solution and loads a programmed volume of sample solution into the injector needle and attached syringe. The autoinjector apparatus then moves the syringe and injector needle position to the injector valve and loads the sample solution into the injector valve. To avoid sample carryover from one sample solution injection to the next, the injection needle inner bore and outer surface and the syringe inner volume may be washed or flushed between sample injections.
Other commercially available autoinjectors do not attach the injector needle directly to a syringe but instead connect the injector needle to a fluid transfer line or tube that is in turn connected to a syringe or fluid pump which may or may not translate with the injector needle position. Sample solutions are drawn into the injector needle and connected tubing and injected into the injector valve by activating the remote syringe or fluid delivery pump when the injector needle is appropriately positioned in a sample vial or the injector valve respectively. A number of apparatus and methods have been employed in commercial autoinjectors to flush or wash the outer and inner bore of injection needles and the connected tubing between sample injections. Typically, autoinjector needles are metal tubes with sufficient rigidity to push through the seal of an injector valve or sample vial top. The flow rate of sample solution pulled into or delivered from the injection needle is programmably controlled by the autoinjector syringe or positive displacement fluid flow pump. Autoinjectors can be programmed to inject sample solutions drawn from multiple sample vials or containers in an unattended sequence. Each sample loaded into the injector valve is subsequently injected or delivered to the API MS where a portion of the sample is ionized and mass to charge analysis. Alternatively, some or all sample solution transfer, injection and injector needle cleaning steps performed by an autoinjector can be performed manually as well with a handheld syringe or a syringe mounted on an syringe pump that is connected through a transfer line and an injection needle to the injector valve.
API MS performance can be reduced using conventional FIA configured with injector valves and transfer lines. MS signal resulting from ES and APCI source ionization is essentially sample concentration dependent. Dilution of the sample can occur in injector valves and transfer lines due to diffusion of sample solution into the mobile solvent, mixing connection points and in dead volumes and adsorption to the walls. Such dilution can result in reduced MS signal or tailing of injection peaks. Sample that has adsorbed to surfaces in the injector valve or liquid transfer lines can bleed off during subsequent sample injections. Such sample carry over can appear as added peaks or chemical noise in subsequent injections and may cause errors in trace component or quantitative MS analysis. The effects of the dead volumes from injection valves, connections and fluid deliver or transfer lines become increasingly pronounced as the liquid flow rate or sample concentration decreases. When the liquid flow rate is decreased, the sample transit time in the injector valve and transfer tubing increases for a given dead volume. Longer sample transit times allow increased sample diffusion into the solvent, diluting the sample. Higher liquid flow rates may require more total sample to be injected to accommodate slower MS data acquisition rates encountered with scanning mass spectrometers such as quadrupoles.
The Electrospray needle in some commercial ES sources is operated at kilovolt potentials during spraying. For such ES sources, a longer dielectric liquid transfer line of several inches is typically configured between the ground potential injector valve and the ES needle to allow a gradual drop in kilovolt potential through the sample solution. A high electric field gradient in the transfer tube is avoided to minimize sample heating, electrophoretic and electrolysis effects during FIA. Liquid transfer lines can be reduced in length when an ES source in configured with a grounded needle, however, even with grounded ES needles, the dead volume due to the transfer lines cannot be entirely eliminated. For API MS FIA applications where small amounts of sample are available for injection, sample dilution or losses due to injector valve, connector and transfer line dead volumes and surfaces may compromise the limit of detection. Sample handling techniques employed in conventional FIA apparatus and methods may be the primary limitation in achieving lower limits of detection in API MS FIA analysis.
The invention reduces or eliminates those elements configured and used in conventional FIA apparatus and methods that reduce API MS FIA performance. In one preferred embodiment of the invention, the injector needle and an ES source has been configured such that the sample solution can be sprayed directly from the injector needle tip. The injector needle tip is introduced into the ES source chamber through a probe that serves as a needle guide, seal, electrical connection and pneumatic nebulization second needle layer. The injector needle can be introduced into an APCI source through a similar probe apparatus serving as a needle guide, seal and pneumatic nebulization sprayer second tube layer. Multiple injection needles can be configured to spray in a multiplexed manner through one or more API probes to increase FIA sample throughput. The injector needle can be configured as a reusable or disposable tip. The liquid spray flow rate is set by the auto or manual injector sample injection flow rate. This flow rate can be set to optimize MS analysis and sample throughput. The invention reduces instrument cost by eliminating the need for an injector valve and controls, transfer lines and a separate solvent flow pump in API TOF FIA. The invention also minimizes solvent consumption and waste.
The invention allows increased sample throughput in API MS FIA applications by eliminating steps and the time associated with liquid transfer per injector needle. In one embodiment of the invention, multiple injector needles can be sequentially introduced into one API source probe or multiple injector needles can be introduced into an API source through multiple API source probes. T. Wang et. al., Proceedings of the 46th ASMS Conference on Mass Spectrometry and Allied Topics, 1034, 1998 have reported the configuration and use of multiple injector needles and valves to shorten analysis run time and increase sample throughput. Commercially available autoinjectors, such as the Gilson Multiprobe 215 liquid handler, have been configured with up to eight autoinjector needles dispensing to eight autoinjector valves which transfer sample through an additional selector valve to an API source. Fluid flow through such a system is provided by a separate liquid flow pump. The transfer lines have increased length from multiple injector valves when compared to the single injector valve configuration. The increased transfer and dead volumes from each injector valve through the transfer lines and the switching valve to the API source must be thoroughly flushed between injections. The speed of injections even with such a multiple injector valve configuration is still limited to some extent by the washing and flushing of the eight injector valves, transfer lines and switching valve. In one embodiment of the invention, multiple injector needles can be configured for introduction into one or more API probes without the need to add multiple injector valves, transfer lines, switching valves or an additional fluid flow pump. Increases in sample throughput can be achieved with the invention at a lower cost, when compared with commercially available systems, without a reduction in performance that is unavoidable in API MS FIA apparatus with higher dead volumes. In the invention internal flushing or cleaning is limited to the injector needle and the attached reservoir and external flushing is limited to the injector needle only to avoid cross talk or contamination sample carry over from one injection to another. Flushing or cleaning of valves or transfer lines is eliminated in FIA according to the invention.